The sixth-seeded Jankovic, from Serbia, served up seven double faults and was successful on only 59 percent of her first serves as Radwanska advanced to the quarterfinals against top-seeded Victoria Azarenka of Belarus. It is just the fourth time this season Radwanska has reached the quarters.
Radwanska, ranked No. 39, has five top-20 wins this season after defeating Jankovic, ranked 16th.
Wild-card entrant Virginie Razzano of France also advanced with a 6-1, 6-4 victory over eighth-seeded Carla Suarez Navarro of Spain. Razzano, who beat two-time Grand Slam singles champ Svetlana Kuznetsova of Russia in the first round, will face third-seeded Petra Kvitova of the Czech Republic.